You have Time Machine snapshots on the drive:
Time Machine Snapshots
Open the Terminal in the Utilities folder and enter or paste the appropriate command line. Press RETURN and enter your admin password when prompted. It will not be echoed. Press RETURN again.
To turn them ON: sudo tmutil enablelocal
To turn them OFF: sudo tmutil disablelocal
Note that turning them OFF will also delete all existing snapshots.
For more about snapshots see: OS X Mountain Lion- About local snapshots.